Given equivalent pricing, which drop-in light would you choose for a water-tight submersible application? Light output needs to be at least 300 lumens (500 desirable) and runtime at least 2 hours using one protected Panasonic 18650 3400mAh battery.
Brightness is very important but run time is critical.
Cree XP-G R5 1-mode Cool White producing 450 lumens at 1.5 amps
Or
Cree XM-L T6 1-mode Cool White producing 450 lumens regulated down to 1.5 amps
Thanks
At 1.5 amps each, the runtime will be the same. The only noticeable difference between the two should be that the xm-l t6 will produce a more floody, diffuse beam of light, while the xp-g r5 will give you a more concentrated throwy-ish beam. I would probably go with more throw in an underwater situation, especially at this light level, but that’s totally personal.
I agree with Legolas. Underwater a lot of the light would be lost and your range is much shorter. The XP-G or XP-G2 would be a better idea.
See I would have thought spotting sharks with a wide beam of light to say 10 metres made more sense than trying to spot them with a laser focused beam going out to 15. 
I don't know where the numbers from the OP came from, but the XM-L produces almost 40% more light than the XP-G, both at 1.5A. The XM-L does that at a voltage that is 0.2V lower, so power-wise it is even a few percent better.
The underwater light compartment is for flounder gigging so the distance is not as important as the sharp wide angle illumination. Ten-twelve feet in 2-3 feet of water is all that is needed.
I thought it was the XP-G with lower vF at a given current. With a linear driver it won’t matter while the light is regulated, it will draw the same amps and burn off the extra voltage but he will get regulation for longer with the lower vF chip.
Then you have your answer, go with the XM-L, but keep in mind either chip would need current limiting, even through the datasheet cuts off at 1.5A the LED chip will take all the amps the battery can give so without circuitry it will draw enough current to burn itself out instantly after you hit the switch.
Use a sinkpad/noctigon mounted chip for maximum output, and you will have great heatsking under water.
I lit up and compared the light throw using an Orbtronic 18650 3400mAh battery using the XM-L T6 regulated down to 1.5 amps an the XP-G R5 regulated at 1.5 amps (as stated by the sellers). The XM-L T6 had an edge on brightness but it heated up within minutes, hot to the touch, while the XP-G R5 remained only warm.
I’m only an endpoint user and have no testing equipment, but if both the drop-ins were regulated to 1.5 amps as the seller indicated, should this be expected?
Heat comes from energy not converted to light, so the XP-G should get warmer (slightly)
Its likely you got an XP-G that is drawing less then 1.5A or the XM-L is drawing more then 1.5A.
Have you measured the current draw for both?